One of the most important areas in the development of synthetic fuel plants based on fluidized-bed gasification technology is wastewater treatment, particularly to remove cyanide. The presence of cyanide is also a problem in other wastewaters, including spent cyanide liquors which are obtained as by-products in several different industrial processes. Although they are sometimes produced in relatively small quantities, their storage, transport, treatment, and disposal present considerable hazards and serious technological difficulties. Examples of spent liquors are spent cyanide solutions from electroplating and metal finishing shops and barren bleed solutions from gold and silver extraction operations. Cyanide-containing waste scrub gases are produced in fluid catalytic cracking processes. Cyanide is found in industrial wastewaters from coke manufacturing and iron making and from coal gasification and liquefaction. At present, little specific information is available to explain mechanisms for the formation of cyanide during coal coking or coal conversion, but some experts believe that under conditions existing during coal carbonization and gasification ammonia released from coal may be converted to cyanide. Its production is enhanced by high temperatures and it may be derived to some extent by pyrolysis of nitrogenous products obtained as a result of coal decomposition, such as from pyridine.
A number of methods are known for chemically detoxifying cyanide-containing wastewaters. The most common methods for cyanide removal include chemical oxidation, ion exchange, and precipitation. Chemical oxidation includes a variety of processes of which alkaline chlorination is the most common. In this process, chlorine is introduced into a cyanide-contaminated wastewater with a pH greater than 8.5 in order to effect the oxidation of free cyanide to cyanate. Further oxidation of cyanate to carbon dioxide and nitrogen occurs if chlorine and caustic soda are added in excess of the quantity for the first stage of the complete reaction. It is important that the pH of the solution be maintained above 8.5 in order to prevent the release of toxic cyanogen chloride gas from solution. Free cyanide also can be destroyed by hydrogen peroxide and ozone treatment. Another process uses sulfur dioxide, either as gas or as sulfite solution, in the presence of air and a catalyst to oxidize cyanide to cyanate. The cyanide oxidation reaction is catalyzed by the presence of copper ions in solution. The catalytic effect of copper is not unique to the SO.sub.2 /air oxidation process. Copper has also shown to improve the kinetics and chemical utilization efficiency during ozonation and hydrogen peroxide treatment of cyanide-containing wastewaters, and to catalyze the oxidation of cyanide on granular activated carbon.
Ferrous iron in the form of ferrous sulfate can be added to a wastewater to convert free cyanide to ferrocyanide. Generally, complexation efficiency increases with increasing pH. However, operation at pH values greater than 9.0 is subject to excessive ferrous hydroxide precipitation. In some instances prussian blue is formed during the reaction. The ferrocyanide is removed from the wastewater by application of selective ion exchange treatment. Poor elution of cyanide complexes from strong base anion resins has resulted in continual loss of capacity through repeated regeneration cycles. In practice, spent regenerant disposal presents a problem because of the possibility that this material may be classified hazardous. Since ion exchange merely concentrates the mass of complexed cyanide in a smaller volume regenerant stream, the process still presents waste disposal problems because of the toxic wastes produced.
Precipitation as a chemical treatment alternative is limited to concentrated cyanide streams. This limitation is due to the solubility of the metal cyanides formed during the precipitation reactions. In general, precipitation alone will not lower the cyanide content in wastewater to a concentration that is acceptable for discharge. The deliberate addition of precipitating agents is not considered a cost effective treatment option because of the toxic sludge that is produced.
The use of polysulfides for treatment of cyanide wastewaters was reported at an early date in Wernlund, U.S. Pat. No. 2,194,438, which was issued Mar. 19, 1940. Polysulfide solutions have not only been used for the treatment of cyanide liquors but have also been used commercially to control cyanide induced corrosion in fluid catalytic crackers and cokers and has been recently adapted to scrub hydrogen cyanide from gases produced in the fluid catalytic cracking process. The use of polysulfides to treat concentrated cyanide solutions from electroplating shops has been reported. It has also been proposed to use polysulfides for treating cyanides present in coal gasification wastewaters. Other typical disclosures of the use of polysulfides are found in Oil and Gas Journal, (Apr. 14, 1980), pp. 150-153; Journal WPCF, Vol. 57, No. 11, (Nov. 1985), pp. 1089-1093; Environmental Science and Technology, Vol. 13, No. 12, (December 1979), pp. 1481-1487; and Journal WPCF, Vol. 51, No. 9, (September 1979), pp. 2267-2282. While the use of polysulfides has been generally successful, there is definite room for improvement in terms of reaction kinetics and conversion efficiency. Moreover, it is necessary to ensure that the wastewaters are converted to a truly non-hazardous condition for discharge into the environment.
It is accordingly an object of this invention to provide a process for the treatment of cyanide-containing liquid effluents to render them non-hazardous which is highly effective, has a high-reaction rate, and has improved efficiency.
It is another object of the invention to provide a process of the character indicated which involves a catalyzed polysulfide treating step.
It is a further object of the invention to provide an improved process for cyanide removal from cyanide containing waste effluents which involves a novel combination of steps leading to improved results.